Rear End Noise

[zedbear]

2000 Sierra Z-71, 3.73's, G80 locker....

 

A few months ago, I changed the rear axle seals as well as the backing plates and also installed an aftermarket diff cover. I refilled with Mobil 1 75w-90. Not long after I noticed a slight hum or whine around 30-50 mph. It goes away when I let off the gas....

 

I used to own a 95 Grand Marquis and I changed the rear diff fluid to the same Mobil 1 stuff and also started to notice a hum or whine but at higher speeds (probably due to the 2.73's it had as opposed to my truck's 3.73's). But the noise was barely noticeable in the car so I never did anything about it and never had a problem.

 

But in the truck, the noise is more annoying and a concern. Could it be the fluid? I went to the dealer today and found the factory stuff to be 35 bucks a quart. But since I don't know its the fluid (or even the rear diff) for sure, I hate to waste time and money changing the fluid for nothing, especially if I use the crazily priced GM stuff.

 

When I had things apart back there, nothing seemed excessively worn. No slop or excessive wear particles.

[bronyaur]

When my Suburban did that it turned out to be the pinion bearing. I could grab the driveline at the pinion and shake it back and forth and there was noticeable slop.

[zedbear]

When my Suburban did that it turned out to be the pinion bearing. I could grab the driveline at the pinion and shake it back and forth and there was noticeable slop.

 

I'll have to check that out, thanks.

[zedbear]

Checked the above and found no play. Switched to royal purple 75w-90 today. Still notice some noise but I still can't tell for sure where it's coming from. I also suspect the transfer case, which I think is on it's way out.